A student is experimenting with a substance that moves into the cell via facilitated diffusion. The student notes that as the concentration of the substance outside of the cell increases, the rate of movement of the substance into the cell increase and then plateaus. What explains the phenomenon seen by the student?
A) Facilitated diffusion requires transport proteins, which will become saturated at high levels of the substance.
B) Facilitated diffusion requires ATP, which will become depleted at high levels of transport across the membrane.
C) Facilitated diffusion requires the movement of water with the substance, and when water becomes limiting, the movement of the substance will stop.
D) Facilitated diffusion occurs only when the cell requires the substance inside of the cell and will stop when the cell no longer needs the substance.
